1. Field of the Invention
The present invention relates to a process for producing a polyethylene film resin or composition. More particularly, the present invention relates to a polyethylene film composition made from a linear copolymer of ethylene and at least one other olefin and to such a composition that improves the bubble stability without any significant reduction in tear strength of film produced from the composition as compared with film produced from the linear copolymer.
2. Description of the Prior Art
High molecular weight, high density polyethylene (HMW HDPE) film resins for tough, thin film applications are generally extruded by a high stalk extrusion process that produces a biaxially oriented film. Economically, competitive high extrusion rates must be employed by the film manufacturer to minimize production costs. Small dies are used to attain the high blow-up ratios required to produce film of sufficient strength for the various thin-film applications. Since the performance requirements for producing tough thin film are very stringent, only HMW HDPE resins, which have broad, bimodal molecular weight distributions (MWDs) are viable in the marketplace. Resins or compositions that do not possess these characteristics do not extrude well at equally high rates because of bubble instability, among other factors, and also produce poorer film quality as compared to the leading polyethylene film compositions.
Workers in the field are constantly seeking to improve the processibility, e.g., the bubble stability, of the resins while maintaining necessary film properties such as Elmendorf tear and Total Energy Dart Drop (TEDD).
Preparation of linear high density copolymers, primarily containing ethylene, using, for example, highly active Ziegler-type catalysts, is well known and widely practiced in the industry. These catalysts, while increasing production rates of the resin, tend to produce polymers having a relatively narrow molecular weight distribution, resulting in processing problems during the high stalk extrusion process.
Desirably, one would combine highly active catalysts to achieve enhanced production rates of resin while at the same time producing a resin that would exhibit excellent bubble stability in high stalk extrusion. To a certain extent, these results have been achieved with what is commonly referred to as a broad bimodal MWD polyethylene, which contains a high molecular weight fraction having a weight average molecular weight of at least 600,000 and a low molecular weight fraction having a weight average molecular weight of less than 20,000. Such a composition generally has a broad enough MWD to be easily processed, so that excessive pressures and temperatures are not necessary, but not so broad that problems are encountered with poor physical properties in film produced from the resin. Nonetheless, there still remains a need for an improved film-grade polyethylene composition that, under high stalk extrusion conditions, exhibits excellent bubble stability without any appreciable loss of physical properties in film produced from the resin.